The present invention relates to an anti-lock brake system with traction slip control comprising a pedal-actuated, auxiliary-force assisted braking pressure generator having a master cylinder with at least two chambers to which wheel brakes are connected by way of brake lines.
In known brake systems of this type (German published patent applications 30 40 561 and 30 40 562), a master cylinder with a hydraulic brake power booster connected upstream thereof is used as a braking pressure generator. The auxiliary-pressure supply system comprises a hydraulic pump and a hydraulic accumulator, out of which auxiliary pressure proportional to pedal force is introduced on brake application with the aid of a control valve. On the one hand, this dynamic pressure is transmitted by way of the master cylinder into the static brake circuits connected to the master cylinder. On the other hand, the wheel brakes of one axle, preferably those of the rear axle, are in direct communication with the pressure chamber into which the pressure proportional to pedal force is introduced through the control valve. For the purpose of slip control, moreover, inlet valves are inserted both into the static circuits and into the dynamic circuit which normally assume their open position and which, in the event of an imminent locked condition of a wheel, shut off the pressure-fluid flow to the wheel brake concerned.
Further, outlet valves are provided which allow discharge of the pressure fluid from the wheel brake to the pressure-compensating reservoir as needed. On commencement of slip control, the booster chamber in which the controlled pressure introduced out of the auxiliary-pressure supply system prevails is connected by way of a so-called main valve with the static brake circuits of the master cylinder in order to replenish the quantity of pressure fluid removed through the outlet valves into the static circuits.
In brake systems of this type, the control signals for the inlet valves and outlet valves are generated by electronic circuits, the inputs of which are connected with wheel sensors, for example, inductive transducer, and which thereby react on a variation of the wheel rotational behavior indicative of an imminent locked condition by maintaining the pressure at the relevant wheel constant, by reducing it and by re-increasing it anew.
Furthermore, a brake system is known from German published patent application 36 35 054, wherein each pump communicates by way of supply lines to the main brake lines. It has been found that the pumps are not in a position to deliver a sufficient amount of pressure fluid at the onset of a traction slip control action. Therefore, it is suggested in the above publication to arrange accumulators at the outlet of the pumps and to furnish the supply lines with shut-off valves which are opened in the event of a traction slip control action or brake slip control action. In the instance that driven wheels are allocated to both brake circuits, this known arrangement connects one accumulator to each supply line. This case occurs for instance when the vehicle is equipped with all-wheel drive or when the wheel brakes of diagonally opposite wheels form a brake circuit (diagonal allotment). It is recognized that the use of two hydraulic accumulators is costly and renders the brake system more expensive.
The present invention has as an object to devise an anti-lock brake system with traction slip control such that only one accumulator is employed, while, however, both brake circuits are simultaneously supplied with a sufficient pressure fluid.